1. Field of the Invention
The present invention relates to an optical sheet by which light between polarizers disposed in the form of crossed-Nicol can be cut off at wide-ranging azimuth angles so that a good-display-quality vertically oriented liquid-crystal display device which is excellent in viewing angle and contrast can be formed.
The present application is based on Japanese Patent Application No. 2000-302175, which is incorporated herein by reference.
2. Description of the Related Art
Even in the case where light between polarizers disposed in the form of crossed-Nicol was cut off in a normal-line (frontal) direction in which an axis of transmission and an axis of absorption functioned normally, there was heretofore a problem that light leakage occurred in a displaced viewing azimuth crossing the optical axis so that the intensity of leakage light increased gradually as the angle of oblique viewing increased. The problem was revealed as a disadvantage that display quality was deteriorated by reduction in display contrast due to light leakage in a viewing azimuth displaced from the optical axis when the polarizers were disposed on opposite sides of the liquid-crystal cell in the form of crossed-Nicol to function as a polarizer and an analyzer to thereby form a liquid-crystal display device.
In a vertically oriented (VA) liquid-crystal cell in which liquid-crystal molecules were oriented approximately vertically with respect to a cell substrate so that light was able to be transmitted with little change of the plane of polarization, when polarizers were disposed in the form of crossed-Nicol on opposite sides of the cell, light was apt to be cut off in a frontal (normal) direction of a display panel perpendicular to the cell substrate in an inoperative mode having no external voltage applied to thereby make it possible to form good black display easily, in comparison with a TN liquid-crystal cell or the like in which liquid-crystal molecules were oriented horizontally with respect to a cell substrate so that display quality was apt to be deteriorated by light leakage owing to birefringence of the liquid-crystal cell at light transmission. However, the problem that contrast was deteriorated by light leakage in an oblique viewing azimuth displaced from the optical axis of the polarizers because of the aforementioned problem caused by the polarizers still remained, even if a phase retarder exhibiting refractive index anisotropy of nx=ny greater than nz as disclosed in Unexamined Japanese Patent Publication Sho. 62-210423 was used in combination with the vertically oriented liquid-crystal cell for compensating for birefringence of the liquid-crystal cell caused by oblique viewing.
An object of the present invention is to develop a member capable of suppressing light leakage even in an oblique viewing azimuth displaced from the optical axis between polarizers disposed in the form of crossed-Nicol to thereby obtain a vertically oriented liquid-crystal display device which is good in display quality and which is excellent in contrast at wide-ranging viewing angles.
According to an aspect of the present invention, there is provided an optical sheet constituted by a retardation film, and a transparent layer provided on one of opposite surfaces of the retardation film. The retardation film exhibits Nz=(nxxe2x88x92nz)/(nxxe2x88x92ny) in a range of from 0.4 to 0.6, both not inclusively, and (nxxe2x88x92ny)d in a range of from 200 to 350 nm in which d is a thickness of the retardation film, nz is a refractive index in a direction of a Z axis expressing a direction of the thickness d of the retardation film, nx is a refractive index in a direction of an X axis expressing a direction of the retardation film in a plane perpendicular to the Z axis while the X axis expresses a direction of the highest in-plane refractive index, and ny is a refractive index in a direction of a Y axis expressing a direction of the retardation film perpendicular both to the Z axis and to the X axis. And the transparent layer has a thickness not larger than 10 xcexcm and exhibits refractive index anisotropy of nx≈ny greater than nz.
According to another aspect of the present invention, there is provided a polarizer constituted by a laminate of an optical sheet as defined above, and a polarizing film. According to a further aspect of the present invention, there is provided a liquid-crystal display device constituted by a vertically oriented liquid-crystal cell, and a pair of polarizers as defined above, the pair of polarizers being provided on opposite sides of the cell in the form of crossed-Nicol. Further, at least one of the polarizers has the above optical sheet and its transparent layer is positioned on corresponding one of opposite sides of the cell. According to a still further aspect of the present invention, there is provided a liquid-crystal display device constituted by a pair of polarizers disposed in the form of crossed-Nicol on opposite sides of the liquid-crystal cell, and a phase retarder disposed between the liquid-crystal cell and one or each of the polarizers, one of the polarizers having the above optical sheet. The phase retarder exhibits refractive index anisotropy of nx≈ny greater than nz, and an absolute value of a thicknesswise retardation defined by a product of {(nx+ny)/2xe2x88x92nz} and a layer thickness of the transparent layer in the polarizer having the above optical sheet is in a range of from 0.5 time to 1.3 times as large as an absolute value of a thicknesswise retardation of the liquid-crystal cell.
When the optical sheet according to the present invention is used as a transparent protective layer or the like, a polarizer can be formed. When the polarizer is used as one of polarizers disposed in the form of crossed-Nicol on opposite sides of a vertically oriented liquid-crystal cell, light can be blocked out in the direction of the optical axis of the polarizer in the same manner as that in the background art while birefringence of the liquid-crystal cell can be compensated for in an oblique viewing direction. Hence, light leakage can be suppressed even in an azimuth displaced from the optical axis. Hence, there can be formed a vertically oriented liquid-crystal display device which is excellent in contrast at wide-ranging viewing angles and which is good in display quality. Because the optical sheet can be used as a transparent layer for protecting a polarizing film, reduction in thickness of the polarizer or the liquid-crystal display device can be attained.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.